Switching power converters are well known in the art and are typically used to convert AC power to DC power, to convert DC power to AC power, or to convert one DC voltage level to another. Power converters are used in electronic equipment to control the power that is delivered to variety of loads. These converters typically use semiconductor based switching elements that are cycled via Pulse Width Modulation (PWM). For example, when used to convert DC power to AC power, PWM alters the width of the conduction waveform of the switching device such that the output voltage will oscillate back and forth between a positive peak voltage to a negative peak voltage.
A simple single-phase power converter is shown in FIG. 1. It is composed of a DC source 101, switching elements 102 and 103, an inductor/capacitor filter 104 (having inductor 105 and capacitor 106), and an AC load 107. Generally, the switching elements 102 and 103 are operated at a PWM frequency well above the corner of the inductor/capacitor filter 104 with a varying duty cycle in order to achieve the desired AC voltage at the load 107.
In various applications, the switching elements 102 and 103 are each a semiconductor switch. A semiconductor switch is a switch that includes a semiconductor material such as silicon, silicon carbide and gallium nitride (which are referred to as silicon-based switches, silicon-carbide-based switches and gallium-nitride-based switches, respectively. Examples of semiconductor switches include insulated gate bipolar transistor (IGBT) switches and metal-oxide-semiconductor field-effect transistor (MOSFET) switch), which can be made of silicon, silicon carbide, gallium nitride, a combination thereof, or other semiconductor material (or materials).
An example of a semiconductor switch is an insulated gate bipolar transistor (IGBT) and a diode in parallel. A configuration of an IGBT switch 200 is shown in FIG. 2. The IGBT switch 200 includes an IGBT 201 and a diode 202. IGBTs are often selected for their ability to switch high voltages and currents and for their relatively low on-state voltage (compared with other silicon-based switches). An IGBT is a three-terminal power semiconductor device (a gate (G), a collector (C), and an emitter (E). IGBTs switch electric power in many modern appliances: electric cars, trains, variable speed refrigerators, air-conditioners and even stereo systems with switching amplifiers.
The converter design is significantly impacted by the power losses that result from the use of semiconductor devices. Cooling and waste heat removal are a major size, weight, and cost adder for any given power capacity.
Switching power converting losses generally arise from conduction loss and switching loss. Conduction losses are the normal losses of the switching device while conducting current when running in a saturated condition. Switching losses are the losses associated with the actions of turning active switching devices on and off. Switching losses occur when there is simultaneously, high voltage across the device and current through it during transitions between on and off. Because switching losses are incurred every time the device turns on and off, higher device frequencies result in greater switching losses.
As shown in FIG. 3, IGBTs suffer from high switching losses due to a high voltage-current product during turn-on stage 303 and turn-off stage 304. FIG. 3 shows the typical IGBT voltage and current characteristics in curves 301 and 302, respectively. The switched losses are somewhat worse during the turn-off stage due to the IGBT current tail 305. Another problem with current IGBT devices is that there are conduction power losses due to the on-state voltage that persists while the device is carrying current, which is effectively manifests itself as an on-state resistance.
Further discussion of losses in switching power converters is described in U.S. Patent Appl. Publ. No. 2006/0220626, (Partridge).
A need thus exists for improved switching elements to reduce the losses that occur in switching power converters and to improve the efficiency of switching power converters.